Kinetics of metabolic transients during exercise

The present article reviews some recent work in the kinetics of mobilization of anerobic and oxidative energy upon the onset and offset of exercise in isolated muscle preparations as well as in intact animals and man. It is concluded that at the onset of a rectangular electric stimulation and/or exercise, anaerobic glycolysis sets in with a variable time delay that depends on work intensity and temperature. The adjustment of muscular blood flow (MBF) upon a square wave of exercise is very fast both in the isolated dog gastrocnemius (t 1/2 =9 s) and in human muscles (t 1/2 = 5 to 10 s). Training increases considerably the speed of adjustment. At work onset, O₂ consumption is rate limited by a single first order reaction the kinetics of which, determined by the half time of the V(O2) on response (t 1/2 V(O2) on) is approximately 17 s both for isolated dog gastrocnemii stimulated with trains of short tetani and for intact running dogs. The t 1/2 V(O2) on varies in man (18 to 90 s) accompanied by a variable transient 'early' lactate accumulation related to the variable depletion of body oxygen stores, t 1/2 V(O2) on is shortest (18 s) for trained muscles that are also characterized by lower 'early' lactate release. t 1.2 V(O2) on of muscles with artificially depleted PC and O₂ stores is 12 to 26 s and reflects the true kinetics of mobilization of oxidative energy. Defects of various steps along the glycolytic pathway can be assessed by measurement of t 1/2 V(O2) on and 'early' lactate.